1. Field of the Invention
The invention relates to an optical sheet.
2. Description of the Related Art
FIG. 1 shows a schematic diagram illustrating a conventional backlight module having a reverse prism sheet. Referring to FIG. 1, the backlight module 100 includes a light source 102, a light guide plate 104, a reflective sheet 106, and a reverse prism sheet 108. The light beams emitted by the light source 102 enter a side surface 104a of the light guide plate 104 and propagate in the light guide plate 104 through total reflection. When the light beams propagating in the light guide plate 104 strike rough surface structures (not shown) formed on a top surface 104b of the light guide plate 104, the total reflection condition is no longer met, and the light beams are allowed to pass through the top surface 104b of the light guide plate 104 and enter the reverse prism sheet 108. Finally, the incoming light beams are refracted through and reflected by the reverse prism sheet 108 to propagate in a direction parallel to a normal of the top surface 104b. 
According to the conventional design, since the light guide plate 104 has a plurality of micro structures such as rough surface structures formed on the top surface 104b, the light beams exiting through the top surface 104b are often gathered up but not dispersed. Besides, since the reverse prism sheet 108 fails to narrow the light-emitting angle measured in X-Z plane, V-shape groves 110 formed underneath the light guide plate 104 are additionally needed to narrow the overall light-emitting angle measured in X-Z plane.